USMLE Step 1


Biochemical Pathways

Homocysteine Metabolism
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Homocysteine Metabolism

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Homocysteine can be metabolized down two different pathways: (1) methylation to methionine, (2) or transsulfuration to cystathionine with the eventual formation of cysteine.

Production of methionine from homocysteine is catalyzed by methionine synthase, which requires the Vitamin B9 (Folate)-derived 5-MTHF and Vitamin B12 (Cobalamin). Therefore, decreases in either folate or cobalamin can lead to increases in homocysteine (see Vitamins B9 and B12 Deficiencies). Notably, 5-MTHF is regenerated by MTHFR, and defects in MTHFR can lead to excess homocysteine and homocystinuria (see Homocystinuria - coming soon).

Production of cystathionine and eventually cysteine occurs through a different set of reactions. Namely, cystathionine synthase combines homocysteine and serine to produce cystathionine, using Vitamin B6 (Pyridoxine) as a cofactor. The cystathionine is then cleaved to produce cysteine.

Find this Homocysteine Metabolism mnemonic and more Biochemical Pathways mnemonics among Pixorize's visual mnemonics for the USMLE Step 1 and NBME Shelf Exams.